1. Field of the Invention
The present invention relates generally to pre-impregnated composite material (prepreg) that is used in making high performance composite parts that are especially well suited for use as aerospace components. The present invention is more particularly directed to prepreg that is used to make aerospace composite parts or structures that must tolerate simultaneous exposure to hot temperatures and wet conditions.
2. Description of Related Art
Composite materials are typically composed of a resin matrix and reinforcing fibers as the two primary constituents. Composite materials are often required to perform in demanding environments, such as in the field of aerospace where the physical limits and characteristics of the composite part or structure is of critical importance.
Pre-impregnated composite material (prepreg) is used widely in the manufacture of composite parts. Prepreg is a combination that typically includes uncured resin and fibers, which is in a form that is ready for molding and curing into the final composite part. By pre-impregnating the fiber reinforcement with resin, the manufacturer can carefully control the amount and location of resin that is impregnated into the fiber network and ensure that the resin is distributed in the network as desired. It is well known that the relative amount of fibers and resin in a composite part and the distribution of resin within the fiber network affect the structural properties of the part.
Prepreg is a preferred material for use in manufacturing load-bearing or primary structural parts and particularly aerospace primary structural parts, such as wings, fuselages, bulkheads and control surfaces. It is important that these parts have sufficient strength, damage tolerance and other requirements that are routinely established for such parts and structures. The nacelle, which surrounds the jet engine, is a unique structural component of the aircraft due to the nacelle's close proximity to a significant heat source and the exposure of the nacelle to exterior environmental elements. Many of the composite parts and structures that are present in the nacelle must be able to tolerate both hot and wet conditions.
The fibers that are commonly used in aerospace prepreg are multidirectional woven fabrics or unidirectional tape that contains fibers extending parallel to each other. The fibers are typically in the form of a bundle of numerous individual fibers or filaments that is referred to as a “tow”. The fibers or tows can also be chopped and randomly oriented in the resin to form a non-woven mat. These various fiber configurations are combined with a carefully controlled amount of uncured resin. The resulting prepreg is typically placed between protective layers and rolled up for storage or transport to the manufacturing facility.
The compressive strength of a cured composite part is dictated by the individual properties of the reinforcing fiber and matrix resin, as well as the interaction between these two components. In addition, the fiber-resin volume ratio, as well as the orientation of the prepreg in the part, are factors which affect compressive strength. In many aerospace applications, it is desirable that the composite part exhibit high compression strength. The open hole compression (OHC) test is a standard measure of the compression strength of a cured composite material.
In many aerospace applications, it is desirable that the composite part or structure exhibit high compression strength under both room temperature/dry conditions and hot/wet conditions. This is particularly important with respect to the composite parts and structures that are located near the jet engine where exposure to both high temperature and moisture is a consideration. However, attempts to keep compression strengths high under hot and wet conditions may result in negative effects on other desirable properties, such as the glass transition temperature of the uncured resin (sub Tg) used to form the prepreg.
The sub Tg of the uncured resin is related to the viscosity of the resin. If the sub Tg is too high, the uncured resin may become too viscous and unsuitable for use in forming a prepreg. Likewise, if the sub Tg is too low, the uncured resin may have a viscosity that is unsuitably low for use as a prepreg resin. Accordingly, any attempt to alter a resin formulation to maximise the compressive strength of a resulting cured composite material under both room temperature/dry conditions and hot/wet conditions, must be weighed against the potential negative impact on the sub Tg of the uncured resin.
Resins that include an epoxy resin are commonly used in many aerospace prepregs. It is known that various combinations of different types of epoxy resins may result in wide variations in the properties of the uncured resin and final composite part. The curing agent used to cure the epoxy resin matrix can also substantially affect the properties of both the uncured resin and the final composite part.
When formulating an epoxy resin for use as the resin matrix in aerospace prepreg, it is difficult to predict if a new or altered combination of epoxy resin types and curatives will negatively or positively alter existing properties of the uncured resin and/or the cured composite part. This makes the process of altering resin formulations to achieve desired combinations of properties particularly problematic. An example of a desired combination of properties is where the uncured resin has a viscosity that is suitable for making prepreg and where the resulting prepreg is suitable for making jet engine nacelle parts and structures that must be able to tolerate hot and wet conditions.
It is also known to add a thermoplastic toughening agent to an epoxy prepreg resin. The toughening agent, such as polyether sulfone (PES) or polyetherimide (PEI), is dissolved in the epoxy resin before it is combined with fibers to form the prepreg. Thermoplastic toughened epoxy resins have been widely used in combination with carbon fiber to make aerospace prepreg. Varying the amount of toughening agent affects the sub Tg and viscosity of the uncured resin as well as properties of the resulting cured composite material.
It also is difficult to predict if altering the amount or type of toughening agent in an existing epoxy prepreg resin formulation will positively or negatively affect one or more properties of the uncured resin and/or the cured composite material. This issue becomes even more complex and unpredictable when altering other resin formulation variables, such as the amount and type(s) of epoxy resin and curing agents. Alterations in the-resin formulation which provide one desired property can result in an undesirable negative effect on another property. For example, a formula alteration that increases the hot/wet OHC of the cured composite material to a desired level may result in a change in the sub Tg of the uncured resin that renders the resin unsuitable for use in making prepreg.
Existing aerospace prepregs are well suited for their intended purposes. However, there still is a continuing need to develop resins that have properties which are suitable for making aerospace prepreg where the prepreg is then used to make engine nacelle parts or structures where the compressive strength of the part or structure is not adversely affected by the hot and wet conditions present in the nacelle environment.